In Greek mythology, anyone who looked at the gorgon Medusa would immediately turn to stone.

Yet Medusa had nothing to do with the stone trees near the Sonoma-Napa boundary outside Santa Rosa.

Instead, all of the "wood" at Wine Country's Petrified Forest became rock through natural processes that took hundreds of thousands of years. Visiting the modest open-air attraction is an intimate lesson in geology; on a self-guided, half-mile tour of the property, I felt as if I were stepping back in time.

Another bonus: Because the specimens were once California redwoods, they are, without question, the largest petrified trees in the world.

The walkabout begins with a science lesson.

As the signs told me, 3.4 million years ago a volcano in the direction of Mount St. Helena erupted and spewed ash and other particles into the air and water. Over time, water laden with these particles seeped down into the gaps left behind by decomposing tree fibers, replacing the wood cell by cell with crystallized minerals.

Through this process - a type of fossilization - each tree became stone. Because the minerals replaced the tree fibers one by one, the petrified trees assumed the original structures of the trees they replaced; perfect replicas of knots, bark and burls, all cast in rock.

Armed with this knowledge, I wandered up a dirt path toward the first specimen. Over a rise, there she lay: A 43-foot-long pine tree on its side in a ditch, appropriately named the "Pit Tree."

I admired the tree from a wooden boardwalk that spanned its trunk. A sign indicated it was the only petrified pine tree found in the forest - interesting, considering this part of the Bay Area certainly isn't wanting for green.

Farther along the trail, I encountered a pile of petrified wood, with a dozen or so anvil-size chunks of stone covered in moss. The wood looked as if a renegade logger had come by one afternoon, felled a tree and sliced it like a sausage roll.

I had to stare at it closely to see that the wood actually is fossilized.

The petrified trees got larger from there, with a cluster of behemoths lurking at the top of the hill.

First came the Giant, a petrified redwood that measures 6 feet across. Next up: the Queen, which is 65 feet long and 8 feet wide. Researchers have used petrified rings in the tree's trunk to date the latter of the two, and have determined that it was 2,000 years old before the eruption more than 3 million years ago.

The Ollie Bockee Trees were next. These giants were named for one of the first proprietors of the forest, a woman who bought the land in 1914 and charged 50 cents per tour. Bockee's offspring still operate the property.

Finally, I came upon the Robert Louis Stevenson Tree. Stevenson, an author and poet who spent a number of well-documented years in California's Wine Country, included the Petrified Forest in his 1883 book, "Silverado Squatters," and was one of the first people to put the place on the map. He also happened to be a huge fan of walking in the woods.

My 45-minute walk ended at the visitor center, a glorified gift shop that sells pieces of petrified wood from elsewhere around Santa Rosa as well as fossils, such as trilobites, and gemstones, such as rose quartz and chunks of sulfur from India.

In the back corner of a side room, a small museum outlined more about plate tectonics, the general geology of Sonoma County and the processes through which wood is petrified. The explanations were as fascinating as they were detailed.

Sorry, Greeks, but your mythological Medusa has nothing on the wonders of Mother Nature. {sbox}